1. Field of the Invention
This invention relates generally to apparatus for alternately delivering pre-pressurized first and second liquids to liquid utilization apparatus at either a constant flow or at a constant pressure, and more particularly relates to apparatus for alternately delivering pre-pressurized solvent and pre-pressurized wash at a constant flow to a liquid system.
2. Description of the Background Art
With regard to liquid chromatography, typical prior art liquid delivery or pumping apparatus for delivering pressurized (typically referred to in the art as pre-pressurized) solvent to a liquid chromatography system for elution typically utilizes a syringe pump for delivering the pre-pressurized solvent to the liquid chromatography system. The typical prior art syringe pump includes a cylinder, piston and associated accessories for operation. The syringe pump cylinder contains a limited quantity of solvent which is pressurized and expelled to the liquid chromatography system upon being connected thereto. This is inherently a batch operation, and upon the limited quantity of solvent in the syringe pump being expelled, the syringe pump must be de-pressurized, refilled and re-pre-pressurized to deliver further solvent to the liquid chromatography system, and this causes the liquid chromatography system to be pressurized, de-pressurized and re-pressurized. This problem of pressurization, de-pressurization and re-pressurization associated with liquid chromatography systems is further compounded when a limited quantity of pressurized solvent is delivered to the system, the system is de-pressurized, a limited quantity of pre-pressurized wash is delivered to the system, the system is de-pressurized and pressurized solvent is again delivered to the system and these operations are further repeated. The major disadvantage to these batch operations is that they can be undesirably time consuming and another major disadvantage is that you expose the liquid chromatography system to continual pressurization, de-pressurization and re-pressurization cycles which is believed, at least to a certain extent, to damage or shorten the life of the liquid chromatography system components, particularly the liquid chromatography column.
Numerous pumping systems and apparatus, particularly those including syringe pumps, are known to the prior art for delivering pressurized solvent to liquid chromatography apparatus systems under isocratic and/or gradient elusion conditions. U.S. Pat. No. 4,347,131 issued Aug. 31, 1982 to Robert Brownlee discloses a liquid chromatographic pump module for liquid chromatography for delivering the mobile phase to a separation column under high pressure and which includes a cylinder containing a plunger forming a syringe pump with a seal disposed between the plunger and the cylinder and which further includes two positive action valves for connecting the output of the pump to either a reservoir of the mobile phase or to a sample injection valve. This patent is hereby incorporated herein as if fully reproduced herein. U.S. Pat. No. 5,637,208 issued Jan. 10, 1997 to Theodore A. Dourdeville discloses a solvent pumping system for chromatography and for continuously delivering fluid at a selected flow rate to a receiving system such as a high-performance liquid chromatography system. The solvent pumping system includes first and second liquid pumping units each comprising a syringe and a valve. The first and second liquid pumping units are in fluid communication with one another and are capable of independent actuation. The individual valves are arranged to isolate either the first syringe or the second syringe, respectively, from the receiving system while at least one of the syringes remains in fluid communication with the receiving system whereby system pressure can be continuously monitored and the selected system flow rate maintained. The pumping system includes a controller which receives inputs from first and second pressure sensors, associated respectively with the first and second liquid pumping units and the controller activates in response to the inputs of the first and second syringes individually and also activates the first and second valves individually such that when fluid communication is established between the isolated syringe and the receiving system substantially no system flow error is said to be produced. The pressure sensor is positioned within each pumping unit permitting a pumping unit which has been refilled off line to perform compression of its cylinder contents without perturbation of the on-line process and which further permits discrimination of the point in the compression cycle where compression has been completed and fluid delivery to the system will commenced.
Another syringe or syringe-type pumping system for delivering solvent to a liquid chromatography system is the Micropro Dual Gradient Pump Model No. HPLC2g available from Eldex Laboratories, Inc., 30 Executive Court, Napa, Calif. 94558-6278, the assignee of the present invention.
Although at least certain of the above-noted prior art syringe pumps for delivering pressurized solvent to a liquid chromatography system for elution are presently commercially acceptable and have worked reasonably well, there still exists a need in the art for a new and improved apparatus which can be embodied as apparatus for alternately delivering pre-pressurized solvent and pre-pressurized wash to a liquid chromatography system at a substantially constant flow and wherein the system experiences substantially no de-pressurization and re-pressurization in switching from the solvent to the wash and back to the solvent.